A fuel cell of a fuel cell system utilizes an electrochemical reaction of a hydrogen-containing fuel with oxygen to form water for generating electrical energy. For this purpose, the fuel cell contains at least one so-called membrane electrode assembly (MEA) as a core component, which is a structure made up of an ion-conducting or proton-conducting membrane and electrodes situated on both sides of the membrane: an anode electrode and a cathode electrode. In addition, gas diffusion layers (GDL) may be situated on both sides of the membrane electrode assembly on the sides of the electrodes facing away from the membrane.
The fuel cell is generally formed by a plurality of membrane electrode assemblies arranged in a stack, the electrical power of the membrane electrode assemblies during operation of the fuel cell being additive. Bipolar plates, also referred to as flow field plates or separator plates, are usually situated between the individual membrane electrode assemblies, and ensure that the membrane electrode assemblies, i.e., the single cells of the fuel cell, are supplied with the operating media, the so-called reactants, and are also generally used for cooling. Furthermore, the bipolar plates provide for an electrical connection in each case to the particular adjoining membrane electrode assemblies.
During operation of the single cells of the fuel cell (single cell: membrane electrode assembly and an associated anode chamber delimited by a bipolar plate, and an associated cathode chamber delimited by a second bipolar plate), the fuel, a so-called anode operating medium, is fed to the anode electrodes via an open flow field of the bipolar plates on the anode side, where an electrochemical oxidation of H2 to H+ takes place with emission of electrons (e−) (H2→2H++2e−). Water-bound or water-free transport of the protons (H+) that are formed, from the anode electrodes ((combined) anode of the fuel cell) in the anode chambers of the single cells to the cathode electrodes ((combined) cathode of the fuel cell) in the cathode chambers of the single cells takes place through the membranes or electrolytes of the membrane electrode assemblies, which electrically insulate and separate the reaction chambers in question (anode chamber-cathode chamber pairs) from one another in a gas-tight manner.
The electrons provided at the anode are conducted via an electrical line and an electrical consumer (electric traction motor, air conditioner, etc.) to the cathode. The cathode electrodes of the cathode are supplied with an oxygen-containing cathode operating medium via an open flow field of the bipolar plates on the cathode side, and a reduction of O2 to O2− takes place with acceptance of electrons (½O2+2e−→O2−). At the same time, the oxygen anions (O2−) formed at the cathode electrodes react with the protons transported through the membranes or electrolytes to form water (O2−+2H+→H2O).
To supply a fuel cell stack, referred to below primarily as a fuel cell, with operating media, the fuel cell stack/fuel cell includes an anode supply on the one hand and a cathode supply on the other hand. The anode supply includes an anode supply path for feeding the anode operating medium into the anode chambers of the fuel cell, and an anode exhaust gas path for discharging an anode exhaust gas out of the anode chambers. Similarly, the cathode supply includes a cathode supply path for feeding the cathode operating medium into the cathode chambers of the fuel cell, and a cathode exhaust gas path for discharging a cathode exhaust gas out of the cathode chambers.
The fuel cell system includes a humidifier for humidifying an operating medium, in particular the cathode operating medium. The humidifier transfers a portion of the moisture of an exhaust gas, in particular the cathode exhaust gas, which originates from the fuel cell, to a dry operating medium, in particular the comparatively dry cathode operating medium, in order to increase the power density and service life of the fuel cell (polymer electrolyte membranes; see below) during operation at high temperatures. Since fuel cells are operated at temperatures of approximately 60° C. to 80° C. and below 120° C., and water is formed in a chemical reaction of hydrogen with oxygen, water is generally also present in liquid form.
Part of the liquid water is utilized in the humidifier for humidifying the comparatively dry operating medium. However, due to high flow velocities the power or utilization factor of the humidifier for liquid water is not high, so that a certain quantity of liquid water once again exits the humidifier downstream. Liquid water may result in damage to a turbine situated in the exhaust gas path downstream from the humidifier. If a turbine is used in the fuel cell system, it is necessary to separate the liquid water in the exhaust gas upstream from the turbine with the aid of a water separator in order to protect the turbine.
An expander, for example the turbine, situated downstream from the humidifier may be protected with the aid of a water separator situated upstream from the expander, the water separator preferably being fluid-mechanically coupled into the exhaust gas path downstream from the humidifier. The liquid water which is separated in the water separator may be collected in a water collector and discharged to the surroundings. This takes place, for example, with the aid of a water discharge button in the vehicle. In addition, a problem with the liquid water upstream from the expander may be at least partially addressed with the aid of a comparatively large humidifier or a design of the humidifier based on the humidifying power, which is not high. Furthermore, when there is a comparatively large amount of liquid water in the exhaust gas, a comparatively high heat of condensation results which must be discharged to the surroundings with the aid of a comparatively large radiator.